1. Field of the Invention
The present invention relates to a trial lens which is used to determine an optical center of a non-single spherical surface type contact lens, such as a multifocal lens having a plurality of visual correcting zones with different refractive powers set therein and a toric lens with a degree of lens power for astigmatism correction added thereto, and also relates to related technology.
2. Discussion of Background
Contact lenses for visual correction are well known. The present invention provides a non-single spherical surface type contact lens with a lens surface in an optical area including a plurality of spherical surfaces or a toric surface, such as: (1) a multifocal lens having a plurality of visual correcting zones with different degrees of spherical lens power; (2) a toric lens with a degree of cylindrical lens power set therein; and (3) a toric-multifocal lens with a degree of spherical lens power having multifocal and a degree of cylindrical lens power combined therein. Application of a contact lens to an eye of a person who has degraded accommodation in eye refractive power due to presbyopia and so on, or has ametropia due to astigmatism, has been considered.
In such a non-single spherical surface type contact lens, it is crucial to position an optical area with respect to a pupil in comparison with a single spherical surface type contact lens which has both lens surfaces substantially formed in a single spherical surface to be used for visual correction of spherical ametropia due to myopia or hyperopia. When a non-single spherical surface type contact lens has an optical area which deviates from an optical center of a pupil of the wearer of the contact lens and the deviation is by a great deviation quantity, an aberration could occur causing the contact lens to fail to exhibit a proper visual correcting ability, and degrading clearness in visibility. When a contact lens has an optical center set at a uniform relative position with respect to a geometric center thereof, the contact lens has, in some cases, a deviation quantity of the optical center thereof with respect to the optical center of a pupil increased to fail to exhibit a satisfactory visual correcting function since the stable position of a contact lens on a pupil varies with individuals.
In order to cope with this problem, it has been proposed that an optometer be used to superimpose a meridian distance measuring pattern over an image with the state of wearing a contact lens being photographed so as to compose an inspection image, so that the stable position of the contact lens on the cornea is measured by the use of the inspection image displayed on a monitor, and the position of the optical center on the contact lens is determined based on the measurement so as to favorably position the optical center with respect to a pupil position, which is disclosed in JP-A-646998 for instance.
Such a position determination method requires an expensive measuring device, such as a camera, a monitor, an image processing processor and so on, creating a problem in cost and because the measuring device is difficult to operate. There is a possibility that the measuring accuracy degrades because of misalignment in superimposing a meridian distance measuring pattern over an image when the wearer is photographed in the state of wearing the contact lens.